This invention relates to bidirectional constant current driving circuits which are capable of driving variable attenuators and the like. The invention uses PIN diodes with constant currents, in both positive and negative directions.
Traditionally, constant current driving circuits supply constant current irrespective of impedance variations of the loads driven by such current. Typically, such a circuit applies a voltage to the base of a transistor, the emitter of which is connected to a resistor. The transistor drives a load which is connected to its collector. This kind of driving circuit is often used because it has a simple structure; however, it also has a disadvantage since it provides a constant current in only one electrical direction.